Considering that hanging an engine is at least 4 or 5 years away I am keeping all options open for this crucial piece of the project. In addition to the bolt-on options I am intrigued by the Corvair conversion that has been largely pioneered by William Wynne. I have seen his engines at Oshkosh and at his hanger in Florida (where I also met Gus Warren). The relatively recent advent of 5th bearing options - providing enhanced prop load carrying capability - provides additional confidence in the ability of this auto engine conversion to provide reliable aircraft power.
Ultimately my engine choice will be based on several factors, many of which I have yet to even consider. So, for lack of a commitment at this time I will be content to continue my research and build my case for the proper power plant selection.